Don't retry a mirror which failed on the first attempt

If the mirror attempt failed and we were able to report an error to the
user, it makes no sense to attempt a retry.  We don't have a way to
abort a mirror attempt yet, so if the user got a setting wrong and it's
failing for that reason, the only recourse they'd have would be to
restart the server.

tests/acceptance/fakes/dest/hello_wrong_size.rb

@@ -6,15 +6,24 @@
 
 require 'flexnbd/fake_dest'
 include FlexNBD
+Thread.abort_on_exception = true
 
 addr, port = *ARGV
 server = FakeDest.new( addr, port )
 client = server.accept
 
 t = Thread.new do
-  client2 = server.accept( "Timed out waiting for a reconnection",
-                    FlexNBD::MS_RETRY_DELAY_SECS + 1 )
-  client2.close
+  # The sender *should not reconnect.*  Since this is a first-pass
+  # mirror attempt, the user will have been told that the mirror failed,
+  # so it makes no sense to continue.  This means we have to invert the
+  # sense of the exception.
+  begin
+    client2 = server.accept( "Timed out waiting for a reconnection",
+                            FlexNBD::MS_RETRY_DELAY_SECS + 1 )
+    client2.close
+    fail "Unexpected reconnection."
+  rescue Timeout::Error
+  end
 end
 
 client.write_hello( :size => :wrong )